There has been significant interest in using higher-order modes of optical fibers for multiplexing data to enhance the bandwidth of transmission. Since various modes display various shapes in the plane of fiber cross section, this is commonly known as spatial division multiplexing (SDM), or mode division multiplexing. To compensate the loss in the fiber link, fiber amplifiers capable of amplifying all the modes of interest are required. SDM transmission links that are based on higher order modes depend on multiple-input, multiple-output (MIMO) signal processing, which demands for small modal dispersion of the link.
Rare-earth doped or nonlinear (Raman) fibers can be made with core sizes large enough to support various modes such as LP01, LP11, LP02, LP21, etc. Since the radial field distribution of these modes are different, the overlap factor Γ, of the electrical field of individual modes with the gain region can differ significantly. The overlap factor is defined as:
  Γ  =            ∫              R        ⁢                                  ⁢        1                    R        ⁢                                  ⁢        2              ⁢                  ∫        0                  2          ⁢          π                    ⁢                        E          2                ⁢        r        ⁢                                  ⁢                  ⅆ          r                ⁢                                  ⁢                              ⅆ            ϕ                    /                                    ∫              0              ∝                        ⁢                                          ∫                0                                  2                  ⁢                  π                                            ⁢                                                E                  2                                ⁢                r                ⁢                                                                  ⁢                                  ⅆ                  r                                ⁢                                                                  ⁢                                  ⅆ                  ϕ                                                                        where, R1 and R2 are the radii of the circular region within which various signal modes experience amplification. In a typical amplifier with a cylindrical doped region, R1 is typically zero.
As Γ depends on the transverse mode of the signal wave, the gain experienced in a few mode amplifier (both the rare-earth and Raman/Brillouin amplifiers) by the different modes becomes different. This difference in gain can be problematic when used to amplify various modes carried by a few-moded fiber in a space division multiplexed (SDM) transmission system. It has not yet been possible to split and/or combine various fiber modes without causing significant losses in order for them to be amplified by separate C-, L-band amplifiers (schemes similar to that used in C+L band signal amplification). Another problem associated with SDM transmission systems involving few-moded fiber links is that the group index ng and thus group delay of each mode can be significantly different.
Therefore, there is a need for a few-mode fiber amplifier or amplifying device that will ensure equal gain to all the modes of the fiber.